mach: adopt uv and avoid system python (#34632)

This allows us to use `uv` for:
1. Installing a pinned Python version
2. Installing the dependency packages using `uv`'s pip compatible interface.
4. Bootstrapping `mach` without a Python installion on the host, using `uv
   run`

This change also introduces a new 'composite' GitHub action to setup
python in the different CI workflows. There is no support for externally
managed python installations and virtual environments. These could be
added in the future.

Fixes #34095, #34547

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

.github/actions/setup-python/action.yml

@@ -0,0 +1,20 @@
+name: Setup Python and uv
+inputs:
+  skip-python-setup:
+    required: false
+    description: "Whether to skip installing python using Github's `setup-python` action"
+    default: false
+runs:
+  using: "composite"
+  steps:
+    # Use the setup-python action to take advantage of the cache. uv will
+    # symlink to this version.
+    - name: Setup system python
+      if: ${{ inputs.skip-python-setup != 'true' }}
+      uses: actions/setup-python@v5
+      with:
+        python-version-file: '.python-version'
+    - name: Install uv
+      uses: astral-sh/setup-uv@v4
+      with:
+        version: "0.5.6"